1. Field of the Invention
This invention relates to an image encoding apparatus and method for encoding an image in the XYZ color space to generate an encoded code stream, and an image decoding apparatus and method for decoding the generated encoded code stream.
This application claims priority of Japanese Patent Application No. 2003-391361, filed on Nov. 20, 2003, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
As a typical conventional image compression system, the JPEG (Joint Photographic Experts Group) system standardized by ISO (International Standards Organization) is used. It is known that the JPEG system uses discrete cosine transform (DCT) and provides good encoded and decoded images in the case where relatively high bits are allocated. However, if the number of coded bits is reduced to a certain value or less, block distortion proper to DCT becomes remarkable and deterioration becomes noticeable as viewed subjectively.
Recently, systems for dividing an image into plural bands by a filter called filter bank made up of a combination of a high-pass filter and a low-pass filter and then encoding each band have been studied eagerly. Of these systems, wavelet transform coding has no problem of block distortion due to high compression as in DCT and therefore it is expected to be a new technique that replaces DCT.
The JPEG-2000 system, completed in international standardization in January 2001, employs a combination of this wavelet transform and high-efficiency entropy coding (bit modeling by bit plane and arithmetic coding). It realizes significant improvement in coding efficiency, compared with the JPEG system (see, for example, the following Patent Reference 1). Moreover, the JPEG-2000 system can handle up to 38 bits (per component) while the JPEG system can handle only up to eight bits.                Patent Reference 1: JP-A-2002-165098        
The JPEG system generally uses YCbCr as its color space, but the JPEG-2000 system, which is an extended system, can handle various color spaces such as sYCC, sRGB, Lab and XYZ as file formats. Particularly, DCI (Digital Cinema Initiatives), which is a digital cinema standardization organization, has almost determined to employ the CIE XYZ color space having the largest color range, and development of a technique for highly efficiently compressing and expanding images in the XYZ color space is expected. However, no such technique has been proposed yet.